A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 comprises a disk stack 12 (illustrated as a single disk in FIG. 1) that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 (or head) mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor 28 which moves the transducer 20 relative to the disk 12. The spin motor 14, and actuator arm assembly 18 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a digital signal processor (DSP), a microprocessor-based controller and a random access memory (RAM) device.
Referring now to the illustration of FIG. 2, the disk stack 12 typically includes a plurality of disks 34, each of which may have a pair of disk surfaces 36. The disks 34 are mounted on a cylindrical shaft and are designed to rotate about axis 38. The spindle motor 14 as mentioned above, rotates the disk stack 12.
Referring now to the illustration of FIGS. 1 and 3, the actuator arm assembly 18 includes a plurality of the transducers 20, each of which correspond to one of the disk surfaces 36. Each transducer 20 is mounted to a corresponding flexure arm 22 which is attached to a corresponding portion of the actuator arm 24 that can rotate about the pivot bearing assembly 26. The VCM 28 operates to move the actuator arm 24, and thus moves the transducers 20 relative to their respective disk surfaces 36.
Although the disk stack 12 is illustrated having a plurality of disks 34, it may instead contain a single disk 34, with the actuator arm assembly 18 having a corresponding single actuator arm 24.
FIG. 4 further illustrates one of the disks 34. Data is stored on the disk 34 within a number of concentric radial tracks 40 (or cylinders). Each track is divided into a plurality of sectors 42. Each sector 42 is further divided into a servo region 44 and a data region 46. The servo regions 44 of the disk 34 are used to, among other things, accurately position the transducer 20 so that data can be properly written onto and read from the disk 34. The data regions 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
FIG. 5 shows portions of tracks 40 for the disk 34 of FIG. 4 drawn in a straight, rather than arcuate, fashion for ease of depiction. To accurately write data to and read data from the data region 46 of the disk 34, it is desirable to maintain the transducer 20 in a relatively fixed position with respect to a given track's centerline 48 during each of the writing and reading procedures (called a track following operation). Tracks n−2 through n+1, including their corresponding centerlines 48, are shown in FIG. 5.
To assist in controlling the position of the transducer 20 relative to the track centerline 48, the servo region 44 contains, among other things, servo information in the form of servo patterns 50 comprised of one or more groups of servo bursts, as is well-known in the art. First, second, third and fourth servo bursts 52, 54, 56, 58 (commonly referred to as A, B, C and D servo bursts, respectively) are shown in FIG. 3. The servo bursts 52, 54, 56, 58 are accurately positioned relative to the centerline 48 of each track 40. Unlike information in the data region 46, servo bursts 52, 54, 56, 58 may not be overwritten or erased during normal operation of the disk drive 10.
As the transducer 20 is positioned over a track 40 (i.e., during a track following operation), it reads the servo information contained in the servo regions 44 of the track 40, one servo region 44 at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 20 and a desired position relative to the track centerline 48. As is well-known in the art, the PES signals are input to a servo control loop (within the electronic circuits 30) which performs calculations and outputs a servo compensation signal which controls the VCM 28 to, ideally, place the transducer 20 at the desired position relative to the track centerline 48.
Vibration of the disk drive can cause the transducer 20 to have an across track vibration, which can effect that ability of the servo control loop to maintain the transducer 20 on a track during a track following operation. The servo control loop can be configured to compensate for a worst-case amount of vibration, within design constraints, that the transducer 20 may experience while in a track following operation on various tracks across the disk 34. However, optimizing the servo control loop for worst-case vibration conditions may provide less optimal performance (e.g., less data throughput than otherwise possible) in non-vibration conditions.
A servo track writer (STW) is used to write servo regions 44, including their corresponding fields, onto the surface(s) of the disks 34 during the manufacturing process. The STW controls the transducers 20 corresponding to each disk surface 36 of the disks 34 to write the servo regions 44. In order to precisely write the servo regions 44 at desired locations on the disks 34, the STW directs each transducer 20 to write in small steps, with each step having a width (i.e., STW step width 72 as shown in FIG. 5). FIG. 5 illustrates the relationship between the STW step width 72 and the pitch 74 of the servo region 44 for a conventional disk drive system.
As used herein, the term “pitch” is the radial distance between centers of adjacent regions on the surfaces 36 of the disks 34. For example, a servo track pitch 74 (shown in the data region 46 of FIG. 5) is the distance between the centers of radially adjacent servo regions 44. In contrast, the term “width” is defined as the radial distance from one end to the other end of a single region. For example, a servo track width 75 (shown in the data region 46 of FIG. 5) is the width from one end to another of a single servo region 44.
For each servo region 44, the servo track pitch 74 is generally equivalent to the servo track width 75. However, for data regions 46, the data track pitch 76 is generally different from the actual data track width (not shown) due to, for example, the presence of erase bands which are typically found on both sides of each data region 46. For simplicity, the effects that reduce the data track width are not shown in the figures. Instead, the data track width is shown to be the same as the data track pitch. Although the data track pitch 76 is illustrated as a constant amount across the tracks 40, it is known to vary the data track pitch, and thereby the data tracks per inch (TPI), across a disk surface.